The invention relates to a process for preparing (6S)-folinic acid and its salts, especially calcium, magnesium, potassium and sodium-folinate.
Folinic acid is N-(5-formyl-(6R,S)-5,6,7,8-tetrahydro-pteroyl)-L-glutamic acid (5-CHO-(6R,S)-H.sub.4 PteGlu). N-(5-formyl-(6S)-5,6,7,8-tetrahydropteroyl)-L-glutamic acid (5-CHO-(6S)-H.sub.4 PteGlu) is the citrovorum factor (=growth-promoting factor for leuconostoc citrovorum).
Folinic acid contains 2 asymmetric centers. Also, due to the synthesis of the folinic acid from folic acid, N-(pteroyl)-L-glutamic acid, the optically active C atom contained in the glutamic acid group is present in the L form while the optically active C atom resulting from the hydrogenation of the double bond in the 5,6 position is present in position 6 in the racemic, (6R,S), form. Synthetic folinic acid (=leucovorin) therefore consists of a 1:1 mixture of two diastereomers.
Leucovorin is finding increasing importance as a pharmaceutical preparation for treating megaloblastic, folic-acid-deficiency, anaemia, as an antidote for intensifying the tolerance of folic acid antagonists especially of aminopterin and methotrexate in cancer therapy (leucovorin rescue) and the treatment of autoimmune diseases such as psoriasis and rheumatoid arthritis, and intensifying the tolerance of certain antiparasites, such as trimethoprim-sulphamethoxazole, in chemotherapy.
In the natural state, for example in the liver, folinic acid is found only in the S form. The biochemical action of leucovorin as a folic acid cofactor is based on its content of 5-CHO-(6S)-H.sub.4 PeGlu. On the other hand the inverse (R) form--5-CHO--(6R)-H.sub.4 PteGlu--is barely metabolized and is slowly eliminated through the urine. It is biochemically inactive. J. A. Straw et al, Cancer Research 44, 3114 (1984).
Many attempts have therefore been undertaken to separate 5-formyl-(6R,S)-5,6,7,8-tetrahydro-pteroyl-L-glutamic acid and synthesize 5-formyl-(6S)-5,6,7,8-tetrahydro-pteroyl-L-glutamic acid asymmetrically and isolate the physiologically active form. D. Cosulich et al, J. Amer. Chem. Soc. 74, 4215-16 (1952), U.S. Pat. No. 2,688,018 have attempted to accomplish the separation by fractional crystallization of an alkaline-earth salt, for example the calcium or strontium salt, of 5-formyl-(6R,S)-5,6,7,8-tetrahydro-pteroyl)-L-glutamic acid from an aqueous solution. The required separation cannot be realized under the conditions published by B. Cosulich et al. In the crystallization of, for example, the calcium salt of 5-formyl-(6R,S)-5,6,7,8-tetrahydro-pteroyl-L-glutamic acid from water at pH 7-8 the pure (6R,S)-form is always obtained again, which can be shown quantitatively by chromatographic analysis on a chiral HPLC column and by reference to the optical rotation. In this case it is immaterial whether the crude or pure calcium salt of 5-CHO-(6R,S)-H.sub.4 -PteGlu is used for the crystallization; the optically pure (6R,S)-form is always recovered. Nor can separation and concentration of the (6S)-form be achieved if the super-saturated aqueous solution of alkaline-earth-(6R,S)-folinate is seeded with authentic alkaline-earth-(6S)-folinate. Until now asymmetric synthesis has therefore remained the only possibility for preparing N-(5-formyl-(6S)-5,6,7,8-tetrahydro-pteroyl)-L-glutamic acid.
The previously known methods for asymmetric synthesis of (6S)-folinic acid are, however, not suitable for the preparation of this compound on a commercial scale. Until the present there have therefore been no commercially viable methods for the preparation of (6S)-folinic acid.
There remains, therefore, the problem of developing a simple, commercially useful and cost-effective method for preparing (6S)-folinic acid and it salts.